1. Field of the Invention
The present invention is directed to a magnetic resonance imaging scanner of the type having a radio-frequency system with transmission/reception antennas that is arranged in a basic field system and in a gradient field system, as well as possibly having a radio-frequency shield, and having guides for a movable patient bed.
2. Description of the Prior Art
As basic components, a magnetic resonance scanner has three assemblies: basic field system, gradient field system and radio-frequency system (RF system). The basic field system serves the purpose of providing a strong, static magnetic field (typically 0.1-4 Tesla). The gradient system supplies a magnetic field adjustable in the low-frequency range to about 1 kHz with a linearly rising or decreasing curve in one or more directions (typically up to 30 mT/m). The RF system provides a magnetic field oscillating in the radio-frequency range at the nuclear magnetic resonance frequency of 42.45 MHZ/T essentially predetermined by the static magnetic field for exciting the nuclear spins and also serving for reception of the signals of the relaxing nuclear spins.
These three assemblies surround the patient to be examined in most magnetic resonance scanners in the sequence: RF system, gradient system and basic field system. The patient must be borne on a bed that can be moved into and out of the cylindrical opening.
A problem in all magnetic resonance scanners that has not been satisfactorily solved is the high noise level generated by the gradient system in the strong basic magnetic field because of the strong Lorentz forces. Additionally, the standard structure wherein the individual components of the RF system are built as segments into a tube containing the gradient coils is complicated in terms of assembly as well as being susceptible to malfunction. The structure of the bore for moving the patient bed into and out of the tube given this type of integration is optically disturbing for the patient and can only be kept sterile by means of extremely complicated measures. Finally, this structure also requires complicated internal cables for components of the RF system that are susceptible to malfunction. A structure that is relatively radially thick overall is thus necessary, as a result of which the free diameter of the basic field system must be enlarged in order to provide a predetermined clear (unobstructed) inside diameter for moving the patient bed. This free diameter (warm bore) of the basic field system, however, largely determines the price of the basic field magnet.
These difficulties also apply to a magnetic resonance scanner of the type disclosed in U.S. Pat. No. 5,197,474. Here, the radio-frequency coil is rigidly connected to the portion of a two-part bed and is inserted in common therewith into the basic field system and the gradient system. The cabling must thereby also be drawn in, which likewise enlarges the free diameter of the basic field system.
In U.S. Pat. No. 4,654,596, the radio-frequency coil is wound onto a carrier sleeve whose length corresponds to the length of the radio-frequency coil winding. This short tube section lies inside the scanner and again impedes the introduction and withdrawal of the patient, particularly because the leads here must again be loosely arranged in the through opening for the patient.
U.S. Pat. No. 4,634,980 discloses a magnetic resonance scanner for head examinations wherein the radio-frequency coil, independently of the patient bed which is introducible into the basic field and gradient system, is displaceable over a tongue-shaped extension of the patient bed that accepts the patient's head. This fashioning cannot be transferred at all to a scanner for whole-body examinations because it would then not be possible at all to move the radio-frequency coil over the patient bed which is movable in the basic field and gradient system.